For me I know I’m deeply immersed when my emotions are engaged. For example, if I actually feel good or bad or guilty about my decisions, actions etc. Latest game that has me hooked emotionally is Kingdom Come Deliverance 2. I feel terrible if I go around killing randoms for no reason. Or if I killed someone during a mission because it was easier to achieve my goal - I’m like that guy didn’t NEED to die in order for me to get X. I think it has to do with the that NPC’s do have a life within the world and their own personalities.